1. Field of the Invention
This invention relates to a semiconductor element such as a field effect thin film transistor, and the like, more particularly to a semiconductor element of which a main part is constituted of a polycrystalline silicon thin film semiconductor layer which is high in behavior characteristics, reliability and stability.
2. Description of the Prior Art
Recently, for providing a scanning circuit portion of an image reading, device for use in image reading such as one-dimensional photosensor made in a continuous length or a two-dimensional photosensor of with an enlarged area, or for providing a driving circuit portion of an image display device utilizing liquid crystal (abbreviated as LC), electrochromic material (abbreviated as EC) or electroluminescence material (abbreviated as EL), it has been proposed to form a field effect thin film transistor by using as the base material a silicon thin film formed on a certain substrate, corresponding in size to the increased area of such portions.
Such a silicon thin film is desired to be polycrystalline rather than amorphous for realization of a large scale image reading device or image display device with higher speed and higher function. The effective carrier mobility .mu.eff of a silicon thin film, used as the base material for forming of a scanning circuit portion of such high speed, high function reading device or a driving circuit portion of an image display device, should be large, but that of the amorphous silicon thin film obtained by the ordinary discharge decomposition method is at most 0.1 cm.sup.2 /V.sec, and it also suffers from the drawback that the drain current is reduced as DC voltage is applied to the gate, whereby there is marked change with lapse of time such as moving of the threshold voltage of the transistor, thus being poor in stability.
In contrast, a polycrystalline silicon thin film has an effective carrier mobility .mu.eff by far greater than an amorphous silicon thin film as can be seen from data practically measured. Theoretically, it is quite probable that there may be prepared a polycrystalline silicon thin film having a further increased value of mobility .mu.eff than that presently obtained.
However, under the present state of the art, a semiconductor element or a semiconductor device having as a main part a semiconductor layer of a polycrystalline silicon thin film prepared by various methods of the prior art cannot exhibit sufficiently desired characteristics and reliability.